What Do I Use

B

baymktg

Joined
Feb 5, 2009
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I am a newbie. I have located some O-1 and A-2. I have a 12" disc grinder and few tools. Heat treating techniques available are questionable. Question: For basic working simplicity which is best steel? For hand finishing what is best choice? For hand sharpening which is best choice? For heat treating, which is the most forgiving?
 
I'd say the biggest difference lies in heat treating. O1 is an oil-hardening steel while A2 is air-hardening. Most commercial heat-treating facilities that I'm familiar with only deal with air-hardened steel.
 
Please explain in simple layman's terms air hardening of A2 and how to temper. Thanks.
 
Okay, I'll throw in some info. I use O-1 primarily and yes, the O means oil hardening. Basically what that mean is that you heat the steel to a little over critical temperature (1500 or so for O-1) and then quench in an oil. This varies by the maker and technique but you'll find some using anything from automatic transmission fluid to actual engineered quench oil designed to bring that 1500 degree piece of steel to below a 1000 degrees in 3 to 4 seconds (for O-1). A2, the A being for air hardening means that you use an "air" quench to cool the steel to a hardened state. Most use 1" aluminum plates to pull the heat from the steel along with a high volume fan. The plates are pressed onto the blade in a clamping type setup on both sides of the blade after heating the blade to critical. This "quenches" the steel and causes it to form the martensite required to harden. Tempering is done in an oven, at around 425-450 degrees for O-1, anything from a toaster oven, kitchen oven or an actual heat treating oven. Hardening makes the steel very brittle. Tempering reduces the hardness a few Rockwell C points (RC) but removes the brittleness and allows the blade to flex slightly instead of shattering.
 
With those two steel choices, O1 will be a much better fit if you have questionable means of heat treating. O1 benefits from a longer soak (say, 15-20 minutes at 1475) before quenching in oil. I've used canola in the past before switching to AAA (an engineered quenching oil for medium speed quenching). Heat the oil to around 130F prior to quench. This lowers viscosity and, when combined with agitating the knife tip to butt or spine to edge, prevent bubbles (a vapor jacket) from forming and insulating the steel. O1 is a deep hardening steel and will produce a servicable knife, though not ideal, from even basic heat treating. Bring the O1 up to non-magnetic, continue to heat it up for a bit, and watch the steel. When all the shadows dancing in the blade dissappear and the blade begins to glow more, you're in the neighborhood. If you can, try and keep the blade at that temperature for several minutes without overheating, then quench smoothly in oil. Once the blade is oil temperature, wipe it down and let it cool all the way to room temperature, then temper at 400-450 for 2 cycles of 2 hours each with a cool to room temperature between.

A2 is a poor choice for basic heat treating as it requires higher temperatures (around 1750-1800F) and needs a hold for 30+ minutes at temp. You really need a controlled and monitored forge or a heat treating oven for A2.

--nathan
 
Agree with what the others have said. However, if you are willing to send the blade out for professional heat treating, most all of the services will take A2, fewer will take O1.
 
Good point, Dan. Outsourcing heat treating until you have the means for more accurate temperature control is a good choice. There's a number of places that will do it, but all except a few only work with air hardening steels.

As far as workability, in the annealed state (prior to heat treating), you won't notice too much of a difference I would think between O1 and A2. I've worked with O1 a good bit, and it works just fine. Not too hard on belts and tools. It can be stubborn to sand after heat treating, but it's managable.

--nathan
 
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